Plasma processing device and a method of plasma process

ABSTRACT

Disclosed is a plasma process apparatus which permits generating microwaves and a magnetic field so as to bring about electron cyclotron resonance and, thus, to generate a plasma which is applied to a semiconductor wafer, comprising microwave generating means for generating said microwaves, microwave transmitting means for transmitting the microwaves, a process chamber having said semiconductor wafer arranged therein, the microwaves being introduced into said process chamber through said microwave transmitting means, process gas supply means for supplying a process gas into said process chamber, and magnetic field generating means for generating a magnetic field within the process chamber. The frequency of the microwave falls within a range between a lower limit of a cutoff frequency determined by the inner diameter of the process chamber and an upper limit of a maximum frequency at which a standing wave of the microwave does not occur on the surface of the object.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 08/713,468, filed Sep. 13, 1996, now abandoned.

BACKGROUND OF THE INVENTION

The present invention relates to a plasma processing device forsubjecting an object such as a semiconductor wafer to a predeterminedprocess such as film formation.

The invention also relates to a method of subjecting an object such as asemiconductor wafer to a predetermined process such as film formation byusing a plasma.

With recent development in enhancing integration density andminiaturization of semiconductor products, plasma processing deviceshave been used in some cases in order to perform processes such as filmformation, etching and ashing in steps of manufacturing thesemiconductor products. In particular, a microwave plasma device tendsto be used, since it can create a stable plasma even in a high vacuumstate with a relatively low pressure of about 0.1 to 10 mTorr. In themicrowave plasma processing device, a high-density plasma is created bycombining microwaves and a magnetic field generated from a ring-shapedcoil.

For example, there is known a conventional microwave plasma devicewherein a plasma generating chamber having magnetic field generatingmeans is provided with a microwave introducing port and an electroncyclotron resonance space is produced. Ions are extracted from theplasma generating chamber, and a process gas in a reaction chamber isactivated by the plasma, thus performing various processes such as filmformation.

FIG. 1 is a schematic diagram showing the structure of such aconventional plasma processing device. In the figure, a process chamber11 is formed of, e.g. aluminum in a cylindrical shape. A table 12 formounting of a semiconductor wafer W as an object to be processed isprovided within the process chamber 11. An upper part of the processchamber 11 is narrowed in a stepwise fashion and a plasma chamber 13 isformed in the upper part. A reaction chamber 14 is formed below theplasma chamber 13.

A ceiling cover 15 of, e.g. quartz for sealing a ceiling portion of theprocess chamber 11 is airtightly provided on the upper part of theplasma chamber 13. The ceiling cover 15 constitutes a microwaveintroducing window 16. A conical taper waveguide 17 is connected in amember to face the microwave introducing window 16. A rectangularwaveguide 18 is connected to a top portion of the taper waveguide 17. Amicrowave generator 19 for generating microwaves of, e.g. 2.45 GHz isprovided on the rectangular waveguide 18. Microwaves generated by themicrowave generator 19 are introduced into the plasma chamber 13 throughthe microwave introducing window 16 via the rectangular waveguide 18 andtaper waveguide 17.

Ring-shaped main coils 20 and sub-coils 21 are disposed outside theplasma chamber 13 of process chamber 11 and below the bottom of thechamber, respectively. Each coil 20, 21 generates a downward magneticfield and thereby a downward mirror field is produced within the entireprocess chamber 11. In this case, the downward magnetic field andmicrowaves are set to meet the condition for electron cyclotronresonance. Specifically, if the frequency of microwaves is 2.45 GHz, themagnitude of the magnetic field is set at about 875 gauss.

Thus, the plasma gas, e.g. argon gas, introduced into the plasma chamber13 is made into a plasma by electron cyclotron resonance caused bysynergetic effect of applied microwaves and magnetic field. Thegenerated plasma activates a process gas, e.g. silane gas and oxygenused as film formation gas, supplied to a lower part of the plasmachamber 13. The activated process gas reacts and a reaction productdeposits on the surface of the wafer as a thin film.

In the meantime, when the condition for electron cyclotron resonance issatisfied, the frequency of microwaves and the magnitude of magneticfield are definitively determined by setting the potential, mass, etc.of charged particles. However, if the frequency of microwaves is set at2.45 GHz, as mentioned above, the main coils 20 and the sub coils 21 forobtaining the corresponding field intensity of 875 gauss become verylarge in size. For example, the weight of the main coil 20 becomes 100Kg or more. Consequently, the cost for the plasma processing deviceincreases, the maintenance work for the plasma processing device istime-consuming and the space for installation of the apparatus cannot bedecreased.

In particular, in the case of a plasma processing device for processing12-inch wafers, the diameter of the table 12 further increases, ascompared to the apparatus for processing 8-inch wafers. Consequently,the size of the coil further increases, and the weight thereof becomes,for example, about 200 Kg. Under the circumstances, with an increase indiameter of the wafer, there is a demand for reducing the size of thecoil.

In addition, when microwaves are supplied into the process chamber 11,an impedance variation of the plasma will inevitably occur due to avariation, etc. in density of the generated plasma. Thus, all microwavepower output from the microwave generator 19 is not supplied into theprocess chamber 11, and some reflection power will occur due tomismatching of impedance.

In this case, an effective power contributing to plasma generation isequal to a difference between the output power and reflection power. Inthe prior art, however, no measure is conducted to control reflectionpower, and only output power is controlled. Thus, different powers maybe supplied to wafers, depending on the impedance state of plasma.Consequently, reproducibility of process may deteriorate.

Furthermore, such reflection power is wasted since it does notcontribute to plasma generation. From the standpoint of powerconsumption, the presence of reflection power is not desirable.

BRIEF SUMMARY OF THE INVENTION

An object of the present invention is to provide a plasma processapparatus with a small-sized electromagnetic coil. Another object of theinvention is to provide a plasma process apparatus capable of reducingreflection power of microwaves. Still another object of the invention isto provide a plasma process method in which a small-sized coil can beused. Still another object of the invention is to provide a plasmaprocess method capable of reducing reflection power of microwaves.

According to an aspect of the present invention, there is provided aplasma process apparatus which permits generating microwaves and amagnetic field so as to bring about electron cyclotron resonance and,thus, to generate a plasma which is applied to an object to beprocessed, comprising:

microwave generating means for generating said microwaves;

microwave transmitting means for transmitting the microwaves;

a process chamber having said object arranged therein, the microwavesbeing introduced into said process chamber through said microwavetransmitting means;

process gas supply means for supplying a process gas into said processchamber; and

magnetic field generating means for generating a magnetic field withinthe process chamber,

wherein the frequency of the microwave falls within a range between alower limit of a cutoff frequency determined by the inner diameter ofthe process chamber and an upper limit of a maximum frequency at which astanding wave of the microwave does not occur on the surface of theobject.

This invention also provides a plasma process apparatus for processingan object by using a plasma produced by electron cyclotron resonancewhich is caused by generating microwaves and producing a magnetic field,said apparatus comprising:

microwave generating means for generating the microwaves;

microwave transmission means for transmitting the microwaves;

a process chamber in which the microwaves are introduced via themicrowave transmission means and said object is disposed;

process gas supply means for supplying a process gas into the processchamber;

magnetic field generating means for generating a magnetic field in theprocess chamber; and

matching means for freely varying the impedance of the microwaves in themicrowave transmission means,

wherein said matching means varies the impedance of the microwaves inthe microwave transmission means so as to substantially eliminatereflection waves from the process chamber.

According to another aspect of the present invention, there is provideda plasma process method using a plasma process apparatus comprisingmicrowave generating means for generating said microwaves, microwavetransmitting means for transmitting the microwaves, a process chamberhaving said object arranged therein, the microwaves being introducedinto said process chamber through said microwave transmitting means,process gas supply means for supplying a process gas into said processchamber, and magnetic field generating means for generating a magneticfield within the process chamber, comprising the steps of:

allowing said microwave generating means to generate microwaves;

introducing said microwaves into said process chamber through saidmicrowave transmitting means; and

allowing said magnetic field generating means to generate a magneticfield within the process chamber so as to permit the microwaves and themagnetic field to bring about an electron cyclotron resonance and, thus,to generate a plasma which is applied to the object,

wherein the frequency of the microwave falls within a range between alower limit of a cutoff frequency determined by the inner diameter ofthe process chamber and an upper limit of a maximum frequency at which astanding wave of the microwave does not occur on the surface of theobject.

This invention also provides a plasma process method using a plasmaprocess apparatus comprising microwave generating means for generatingthe microwaves, microwave transmission means for transmitting themicrowaves, a process chamber in which the microwaves are introduced viathe microwave transmission means and an object is disposed, process gassupply means for supplying a process gas into the process chamber,magnetic field generating means for generating a magnetic field in theprocess chamber, and matching means for freely varying the impedance ofthe microwaves in the microwave transmission means, said processcomprising the steps of:

generating the microwaves by said microwave generating means;

introducing the microwaves into the process chamber via the microwavetransmission means; and

producing the magnetic field in the process chamber by the magneticfield generating means, and causing electron cyclotron resonance of themicrowaves and the magnetic field, thereby producing a plasma andprocessing the object with use of the plasma,

wherein said matching means varies the impedance of the microwaves inthe microwave transmission means so as to substantially eliminatereflection waves from the process chamber.

In the plasma process apparatus and plasma process method of the presentinvention, the frequency of the microwave applied to the object isdefined to fall within a range between a lower limit of a cutofffrequency and an upper limit of a maximum frequency at which a standingwave of the microwave does not occur on the surface of the object. Theparticular definition permits lowering the intensity of the magneticfield satisfying the condition of the electron cyclotron resonance,making it possible to miniaturize the electromagnetic coil. As a result,the plasma process apparatus can be miniaturized, leading to saving ofthe space.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 schematically shows a conventional plasma processing device;

FIG. 2 schematically shows an embodiment of a plasma processing deviceaccording to the present invention;

FIG. 3 is a perspectively view showing an EH tuner as an example ofmatching means of the plasma processing device shown in FIG. 2;

FIG. 4 is a cross-sectional view of the EH tuner shown in FIG. 3; and

FIG. 5 shows how a standing wave is formed on a semiconductor wafer.

DETAILED DESCRIPTION OF THE INVENTION

An embodiment of the present invention will now be described withreference to the drawings.

FIG. 2 is a cross-sectional view showing an example of a plasma processapparatus according to the present invention, FIG. 3 is a perspectiveview showing a microwave introducing system of the plasma processapparatus shown in FIG. 2, and FIG. 4 is a cross-sectional view showingmatching means.

In this embodiment, an electron cyclotron resonance (ECR) plasma CVDapparatus is used as an example of the plasma process apparatus. Theplasma process apparatus 30 shown in FIG. 2 has a process chamber 31.The process chamber 31 is formed substantially of, e.g. aluminum and hasa cylindrical shape.

A table 32 formed of, e.g. aluminum for mounting of a semiconductorwafer W as an object to be processed is disposed at a bottom portion ofthe process chamber 31. An electrostatic chuck 34 formed of, e.g.polyimide resin in which a disk-like copper foil 33 is buried isattached to an upper surface of the table 32. A DC power supply 36 isconnected to the copper foil 33 via a power line 35. A switch 37 isprovided on the power line 35. A bias radio-frequency power supply 38of, e.g. 13.56 MHz is connected in parallel to the DC power supply 36via a matching box 39. When the switch 37 is turned on, a high DCvoltage is supplied to the electrostatic chuck 34 and the wafer W isattracted and held by a produced coulomb force. A bias radio-frequencypower is applied to the electrostatic chuck 34, thereby efficientlyattracting ions.

A cooling jacket 40 for cooling the wafer W is provided within the table32, thereby to prevent the wafer W from being excessively heated at thetime of plasma process. The jacket 40 is connected to a refrigerantsource 41. A heater 42 for heating the wafer W, where necessary, isprovided within the table 32. The heater 42 is connected to a heatingsource 43.

A reaction chamber 44 having an open ceiling is formed above the table32 arranged within the process chamber 31. A ceiling cover 46 formed ofa dielectric body such as aluminum nitride (AlN) is hermeticallyprovided on the ceiling portion via a seal member 47 such as an O-ring,thus constituting a microwave introducing window 49 through whichmicrowaves 48 pass.

A conical taper waveguide 50 is connected to the microwave introducingwindow 49. The waveguide 50 is connected to a rectangular waveguide 52having a rectangular cross section via a conversion waveguide 51 forconverting the vibration mode of the microwave. Further, the rectangularwaveguide 52 is connected to a microwave generator 53 via a matchingcircuit 69 for performing an impedance matching. It follows that themicrowave generated within the microwave generator 53 is introduced intothe reaction chamber 44.

The frequency of the microwave generated from the microwave generator 53is set to fall within a range between a lower limit of the cutofffrequency determined by an inner diameter L1 of the reaction chamber 44and an upper limit of a maximum frequency at which a standing wave ofthe microwave does not occur on the surface of the semiconductor waferW. It should be noted concerning the lower limit of the frequency rangegiven above that the frequency of the microwave is proportional to theintensity of the magnetic field under the state satisfying the electroncyclotron resonance conditions. Naturally, a magnetic field of a higherintensity is required with increase in the frequency of the microwave,making it necessary to use a larger electromagnetic coil. Also, theweight of the coil is increased with increase in the size of the coil.

Further, if a standing wave of the microwave is formed on the surface ofthe wafer, distribution of electric field is brought about on the wafersurface, with the result that the wafer cannot be processed uniformly.What should also be noted is that, if the frequency of the microwave islower than the cutoff frequency determined, the microwave cannot beintroduced into the process chamber 31 and, thus, an electric powercannot be supplied to the process chamber 31. In the present invention,the frequency of the microwave is determined as described previously inview of these contradictory requirements.

The upper limit in the frequency of the microwave is 1.5 GHz where thewafer is sized at 8 inches and 1.0 GHz where the wafer is sized at 12inches. On the other hand, the lower limit in the frequency of themicrowave depends on the wafer size and oscillation mode of themicrowave. Where the wafer is sized at, for example, 8 inches, the lowerlimit is 580 MHz under the microwave oscillation mode of TE 11 mode and770 MHz under TM 01 mode. Where the wafer is sized at 12 inches, thelower limit is 440 MHz under TE 11 mode and 570 MHz under TM 01 mode.Clearly, the frequency of the microwave used in the present invention ismarkedly lower than that of the microwave widely used in this technicalfield, i.e., 2.45 GHz.

In the embodiment shown in the accompanying drawings, the industrialfrequency of 915 MHz, which falls within the range specified in thepresent invention in respect of the wafer sizes exemplified above, isused as the frequency of the microwave. Also, the inner diameter L1 ofthe process chamber 31 is set to be larger in general by about 10 cmthan the wafer size. For example, the inner diameter L1 is set at about300 mm where the wafer is sized at 8 inches and at about 400 mm wherethe wafer is sized at 12 inches.

A main electromagnetic coil 54 is wound about the outer surface in anupper portion of the process chamber 31 such that the reaction chamber44 is defined in the upper portion of the process chamber 31 by themagnetic fluxes generated from the coil 54. In addition, with thereaction chamber 44 interposed, the annular sub-electromagnetic coil 55is provided below the bottom portion of the process chamber 31. Thecoils 54 and 55 produce a downward mirror magnetic field M1 within theplasma chamber 44 and reaction chamber 44, thereby effectively enclosingions. With the magnetic field M1 and supplied microwaves, electroncyclotron resonance occurs to generate a plasma.

A plasma gas introducing nozzle 56 is provided on the size wall definingthe plasma chamber 44. The nozzle 56 is connected via a gas passage 57to an Ar gas source 58, an oxygen gas source 59 and a cleaning gassource, e.g. NF₃ gas source 59. The gas sources 57, 58 and 59 areprovided with opening/closing valves 61A, 61B and 61C and mass-flowcontrollers 62A, 62B and 62C, respectively, so that the flow rates ofgases from the gas sources 57, 58 and 59 can be controlled.

In addition, a process gas introducing nozzle 63 is provided on the walldefining the reaction chamber 44. The nozzle 62 is connected to aprocess gas source, e.g. silane gas source 65 via a gas passage 64. Theflow rate of the process gas is controlled by an opening/closing valve61D and a mass-flow controller 62D provided midway along the gas passage64. An exhaust port 66 for creating a vacuum within the process chamber31, which is connected to a vacuum pump (not shown), is made in the sidewall of the reaction chamber 44 of process chamber 31. The side wall ofthe reaction chamber 44 is connected to a load/lock chamber 68 via agate valve 67.

On the other hand, matching means 69, reflection power measuring unit 70and an output power measuring unit 71 are provided on the rectangularwaveguide 52 successively from the plasma chamber (44) side. Thematching means 69 adjusts the impedance of the rectangular waveguide 52in order to eliminate power of microwaves reflected from the plasmachamber 44 ("reflection waves" hereinafter), that is, reflection power.The reflection power measuring unit 70 measures reflection power. On theother hand, the output power measuring unit 71 measures microwave poweroutput from the microwave generator 53, i.e. output power. The microwavegenerator 53 is connected to the rectangular waveguide 52 via aninsulating member 72 for preventing reflected microwaves from enteringthe plasma generator 52.

The matching means 69 is electrically connected to a control section 73comprising, e.g. a microcomputer. The control section 73 controls thematching means 69 on the basis of a measurement value obtained from thereflection power measuring unit 70.

Specifically, an EH tuner 100, as shown in FIGS. 3 and 4, can be used asmatching means 69. The EH tuner 100 serving as matching means 69comprises an electric field plane adjusting unit 101 for adjusting theelectric field (E) plane of the rectangular waveguide 51, and a magneticfield adjusting unit 102 for adjusting the magnetic field (H) planeperpendicular to the electric field plane. The respective units 101 and102 have an electric field adjusting pipe 103 and a magnetic fieldadjusting pipe 104 with rectangular cross sections, which are branchedto communicate with each other perpendicularly to the rectangularwaveguide 51. Short plates 105 and 106 are provided in the adjustingpipes 103 and 104 respectively such that the short plates 105 and 106are put in contact with the inner walls of the respective pipes 103 and104 so as to be movable in the longitudinal direction. Accordingly, theshort plates 105 and 106 are properly moved within the adjusting pipes103 and 104, thereby adjusting the electric field plane and magneticfield plane and varying the impedance.

The short plates 105 and 106 are coupled to slide arms 107 and 108,respectively. The arms 107 and 108 are coupled to an electric fieldplane moving mechanism 109 and a magnetic field plane moving mechanism110. The moving mechanisms 109 and 110 comprise, respectively, racks 111and 112 provided at end portions of the arms 107 and 108, pinions 113and 114 meshed with the racks 111 and 112, and step motors 115 and 116serving as drive means for rotating the pinions 113 and 114 forwardlyand reversely.

As is shown in FIG. 3, the moving mechanisms 109 and 110 adjust thepositions of the short plates 105 and 106 in response to commands fromthe control section 73 so that reflection power becomes substantiallyzero. In this case, in order to obtain optimal positions of the shortplates, a plasma is actually created in advance and the respective shortplates are moved bit by bit. The positions at which reflection powerbecomes substantially zero are plotted for mapping. The mapping data isstored, e.g. in a storage (not shown) in the control section 73. Thestructures of the moving mechanisms 109 and 110 are not limited to theabove-described, if the positions of the short plates 105 and 106 can beadjusted.

A directional coupler, for example, can be used as reflection powermeasuring unit 70.

A measurement value obtained in the output power measuring unit 71 maybe input to the control section 73. On the basis of the measurementvalue, the control section 73 controls the output of the microwavegenerator 53 so that an effective power or a difference between outputpower and reflection power, that is, power actually input to the processchamber 31, may be set at a predetermined value.

A description will now be given of the steps of processing thesemiconductor wafer W with use of a plasma by means of the plasmaprocess apparatus 30 according to the present embodiment having theabove structure.

A non-processed semiconductor wafer W with a diameter of 12 inches iscarried from the load/lock chamber 68 into the reaction chamber 44 ofprocess chamber 31 and placed on the table 32. The wafer W is attractedand held on the table 32 by a coulomb force of the electrostatic chuck34. The process chamber 32 is hermetically closed and evacuated. If theinside of the process chamber 31 has reached a predetermined degree ofvacuum, the Ar gas, O₂ gas and the material gas or silane gas aresupplied into the process chamber 31 from the respective gas sources 57,58 and 59, and the process chamber 31 is kept at a predetermined processpressure, e.g. about 1 mTorr.

At the same time, microwaves generated from the microwave generator 53are propagated through the rectangular waveguide 52 and taper waveguide50 and introduced into the plasma chamber 44 via the microwaveintroducing window 49. Further, the main electromagnetic coil 54 andsub-electromagnetic coil 55 are driven to produce a mirror magneticfield M1 directed downward within the process chamber 32.

Synergetic effect of the mirror magnetic field M1 and suppliedmicrowaves 48 causes electron cyclotron resonance. Thereby, argon gas inthe plasma chamber 44 is made into a plasma, and ions are generated. Thegenerated ions are supplied toward the reaction chamber 44 along thedownward mirror magnetic field M1. The oxygen and silane gas areactivated by the plasma energy of the ions. Thus, sputtering is effectedon the surface of the semiconductor wafer W and at the same time a filmof SiO₂ is formed on the surface of the wafer W.

At this time, a bias voltage is applied from the bias radio-frequencypower source 38 to the copper foil 33 in the electrostatic chuck 34.Thus, ions are efficiently attracted onto the surface of thesemiconductor wafer W.

The plasma-state charged particles absorb microwaves resonating withcyclotron frequency so as to make a circular motion and to be attractedtoward the wafer. In this embodiment, the frequency of the microwave isset to fall within a range between a lower limit of a cutoff frequencydetermined by the inner diameter L1 of the process chamber 31 and anupper limit of a maximum frequency at which a standing wave of themicrowave does not occur on the wafer surface. It is possible to set theparticular frequency at, for example, 915 MHz, which is an industrialfrequency and is markedly lower than the frequency, 2.45 GHz, of themicrowave used in the conventional apparatus. In addition, the plasmadensity is hardly lowered in the present invention in spite of the useof the industrial frequency noted above. It should be noted that themagnetic field intensity meeting the electron cyclotron condition forthe frequency of 2.45 GHz (wavelength of 122 mm) is about 875 gauss. Onthe other hand, the magnetic field intensity meeting the electroncyclotron condition for the frequency of 915 MHz (wavelength of 329 mm)is about 326.8 gauss. Since the frequency of the microwave defined inthe present invention permits markedly lowering the magnetic fieldintensity as pointed out above, it is possible to diminish the mainelectromagnetic coil 54 and the sub-electromagnetic coil 56.

The frequency range of the microwave defined in the present invention isvery important. As already pointed out, the cutoff frequency determinedby the inner diameter L1 of the process chamber 31 constitutes the lowerlimit of the frequency range in the present invention. In order topermit an effective absorption of the microwave for the plasmageneration, it is necessary to transmit the microwave deep into theprocess chamber 31 to reach a horizontal level 84, which is called anECR plane and at which is positioned the lower end of the mainelectromagnetic coil 54. It is ideal for the microwave transmittedthrough the tapered waveguide 50 to pass through the ceiling cover 46made of AlN so as to be introduced into the process chamber 31 such thatthe microwave transmitted deep into the process chamber 31 is to beabsorbed substantially completely at the ECR plane 84. It follows thatthe upper portion of the process chamber 31 forming the reaction chamber44 is required to perform the function of a waveguide. In other words,the microwave must be transmitted through the reaction chamber 44without attenuation. To enable the microwave to be transmitted throughthe reaction chamber 44 without attenuation, the frequency of themicrowave must be higher than the cutoff frequency.

The cutoff frequency is dependent on the inner diameter of a circularwaveguide, i.e., waveguide having a circular cross section, and on theoscillation mode of the microwave. In this embodiment, the processchamber 31 performs the function of a circular waveguide. It followsthat the cutoff frequency fc, which is dependent on the inner diameterL1 of the process chamber 31 and on the oscillation mode of themicrowave, can be represented by formula (1) given below:

    fc=1/λc=1/(A×a)                               (1)

where, "λc" is a cutoff wave length; "A" is a waveguide constant whichis dependent on the oscillation mode; and "a" is the inner diameter ofthe waveguide. In this embodiment, a=L1/2.

Incidentally, the waveguide constant noted above is referred to as λc/ain Appendix 5 (item 247) of "Microwave Circuit" published by NikkanKogyo Newspaper Inc. on Feb. 28, 1969. According to Appendix 5 notedabove, the waveguide constant is 3.413 where the microwave is oscillatedin TE 11 mode, and 2.613 where the microwave is oscillated in TM 01mode.

Suppose the inner diameter L1 of the process chamber for processingwafers each sized at 8 inches (about 20 cm) is set to be larger by about100 mm than the wafer diameter. In this case, the inner diameter L1 is0.3 m. In this case, the cutoff frequency fc in this case is about 580MHz where the microwave is oscillated in TE11 mode and about 770 MHzwhere the microwave is oscillated in TM 01 mode.

Suppose the inner diameter L1 of the process chamber for processingwafers each sized at 12 inches (about 30 cm) is set to be larger byabout 100 mm than the wafer diameter. In this case, the inner diameterL1 is 0.4 m. The cutoff frequency fc in this case is about 440 MHz wherethe microwave is oscillated in TE 11 mode and about 570 MHz where themicrowave is oscillated in TM 01 mode. The cutoff frequency noted aboveis the lower limit of the frequency range specified in the presentinvention.

In the above description, the inner diameter L1 of the process chamberis set to be larger by about 100 mm, which is an allowance, than thewafer diameter. However, the allowance can be set either larger orsmaller than 100 mm. Of course, the lower limit of the frequency rangeis changed in accordance with changes in the allowance in question. Asapparent from formula (1), the lower limit of the frequency range isincreased with decrease in the allowance, and vice versa.

Let us explain the upper limit of the frequency range specified in thepresent invention with reference to FIG. 5. If a standing wave 86covering a length equal to a single wavelength thereof is formed on thewafer surface as shown in the drawing, an electrical potentialdistribution is brought about on the wafer surface. As a result,nonuniformity in the plasma processing is rendered serious so as toimpair the uniformity in the thickness of the film formed on the wafersurface. Naturally, it is important to prevent a standing wave coveringa length of a single wavelength thereof from being formed on the wafersurface. The frequency of the microwave effective for preventingformation of the particular standing wave is 1.5 GHz where the wafer issized at 8 inches, and is 1.0 GHz where the wafer is sized at 12 inches.Of course, the frequency noted above constitutes the upper limit of thefrequency range specified in the present invention. Needless to say, amicrowave having a frequency of 915 MHz, which is the industrialfrequency referred to previously, can be used satisfactorily in respectof the wafers sized at 8 inches and 12 inches.

Where the frequency of the microwave is set at 915 MHz, the lower limitin the diameter L1 of the process chamber, which is calculated byformula (1) referred to previously, is about 25 cm when the microwave isoscillated in TM 01 mode.

As described above, the frequency of the microwave used in the presentinvention can be set markedly lower than 2.45 GHz as in the prior art,making it possible to lower the magnetic field intensity meeting the ECRresonance conditions. It follows that the present invention permitsminiaturizing the electromagnetic coils 54 and 55.

In the present invention, a microwave of any frequency can be usedeffectively if the reaction chamber is sealed sufficiently, as far asthe frequency falls within the particular range. In other words, thefrequency need not be limited to the industrial frequency of 915 MHz.With decrease in the frequency within the particular range, the requiredmagnetic field intensity can be lowered, leading to miniaturization ofthe electromagnetic coils used in the plasma process apparatus.

In general, all microwaves generated by the microwave generator 53 arenot input to produce a plasma. The impedance of the plasma varies, andpart of power is reflected and rendered non-effective in accordance withthe reflection coefficient of the transmission system including theplasma. However, in the plasma process using the plasma processapparatus 30 according to the present embodiment, the electric fieldplane and magnetic field plane are adjusted by the matching means 69provided on the rectangular waveguide 51, thereby to vary the impedanceof the rectangular waveguide 51. Accordingly, it is possible to vary theimpedance of the rectangular waveguide 52 by the matching means 69,thereby reducing the reflection power to substantially zero and makingthe impedance of the rectangular waveguide 52 substantially equal to theimpedance of the plasma. In other words, it is possible to effectimpedance matching, thereby reducing the non-effective output ofmicrowaves to substantially zero.

More specifically, as mentioned above, the matching data, which has beenobtained by displacing the positions of the short plates 105 and 106 ofthe magnetic field plane adjusting units 101 and 102 and measuring thereflection power during this time, is stored in the control section 73.In operation of the apparatus, the control section 73 determines thepositions of the short plates 105 and 106 on the basis of the mappingdata, and the short plates 105 and 106 are moved to the determinedpositions by the electric field plane moving mechanisms 109 and 110.

In this case, the reflection mode of microwaves may become differentfrom that at the time of mapping owing to errors or various conditions.Thus, the presence/absence of reflection power is measured by thereflection power measuring unit 70 at all times, and the measurementsignal is input to the control section 73 as feedback signal. Based onthe feedback signal, the control section 73 further adjusts thepositions of the short plates 105 and 106, thereby effecting impedancematching more exactly and eliminating reflection power without fail.According to the plasma process method using the plasma processapparatus 30 of this embodiment, since the occurrence of non-effectivepower noncontributing to the making of a plasma can be reduced as muchas possible, useless power consumption is reduced and the power issaved.

In the plasma process apparatus 30 of this embodiment, the reflectionpower is monitored by the reflection power measuring unit 70 and at thesame time the output power from the microwave generator 53 is measuredand monitored by the output power measuring unit 71 at all times. Inthis case, since the effective power input to the process chamber 31 isequal to a difference between the output power and reflection power, thecontrol section 73 controls the microwave output from the microwavegenerator 53 so that the effective power becomes constant during theprocess. Even if reflection power occurs, the effective power can,therefore, be made constant at all times, and the reproducibility ofprocess mode is enhanced. Accordingly, uniform processing is achievedamong wafers W. In the case of this embodiment, uniform thickness ofSiO₂ films can be obtained.

As the film forming process is continued, a great deal of SiO₂ filmadheres uselessly to such parts as the inner walls of the reactionchamber 44 of process chamber 31. The excess SiO₂ film, if removed, willresult in particles. To solve this problem, a cleaning operation forremoving excess SiO₂ film may be performed by flowing a cleaning gassuch as NF₃ into the process chamber 31. In this case, if the ceilingcover 46 constituting the microwave introducing window 49 is formed ofquartz (SiO₂), not only the unnecessary SiO₂ film is removed but alsothe ceiling cover 46 is cut in a dome shape by NF₃ gas.

If the ceiling cover 46 is cut in a dome shape and deformed, the mode ofpropagation of microwaves into the process chamber 31 slightly varies,resulting in degradation of reproducibility of process mode.

In the present embodiment, it is desirable that an insulating materialhaving a low etching rate to a fluorine-based gas, a proper dielectricconstant and a relatively high heat conductivity be used as material ofthe ceiling cover 46. Examples of such material are aluminum nitride(AlN) and alumina (Al₂ O₃). In particular, AlN is excellent inconsideration of the above properties.

If AlN is used as material of the ceiling cover 46, it is not so greatlycut by the cleaning gas of NF₃. Thus, the frequency of replacement canbe reduced and the degree of deformation is low, as compared to the caseof using quartz. Therefore, the reproducibility of process mode is good,and high uniformity of processing among wafers W can be maintained. Inaddition, since AlN has relatively high heat conductivity, heat isefficiently transmitted to the process chamber 31 and no conspicuousdeviation arises in a heat distribution within the process chamber 31.

In the present embodiment as described above, silane is used as filmformation gas. The film formation gas is not limited to this and may bedisilane, etc. Besides, the seed for film formation is not limited toSiO₂ and may be SiN₄.

In the plasma process apparatus 30 according to the present embodiment,a plasma is produced by mirror magnetic field. This invention, however,may be applied to an apparatus for producing a plasma by cusp magneticfield.

In the present embodiment, an ECR type plasma CVD apparatus isexemplified as plasma process apparatus 30. This invention, however, isapplicable to, e.g. a plasma ashing apparatus or a plasma etchingapparatus.

The object to be processed by the plasma process apparatus of thisinvention is not limited to the semiconductor wafer W, and may be an LCDsubstrate, for instance.

We claim:
 1. A plasma process apparatus which permits generatingmicrowaves and a magnetic field so as to bring about electron cyclotronresonance and, thus, to generate a plasma which is applied to an objectto be processed, comprising:microwave generating means for generatingsaid microwaves; microwave transmitting means for transmitting themicrowaves; a process chamber having said object arranged therein, themicrowaves being introduced into said process chamber through saidmicrowave transmitting means; process gas supply means for supplying aprocess gas into said process chamber; and magnetic field generatingmeans for generating a magnetic field within the process chamber,wherein the frequency of the microwave falls within a range between alower limit of a cutoff frequency determined by the inner diameter ofthe process chamber and an upper limit of a maximum frequency at which astanding wave of the microwave does not occur on the surface of theobject, wherein the upper limit and the lower limit in the frequencyrange of the microwave are 1.5 GHz and 580 MHz, respectively, where thediameter of the object to be processed is 8 inches and the microwaveoscillates in TE 11 mode within the process chamber, wherein themicrowave transmitting means is provided with matching means for freelyvarying the impedance of the microwaves in the microwave transmittingmeans, and wherein said matching means comprises first and secondadjustment pipes connected to said transmitting means having adjustablerespective first and second plates disposed perpendicular to each other.2. The apparatus according to claim 1, wherein the microwave has afrequency of 915 MHz.
 3. The apparatus according to claim 1, wherein themicrowave transmitting means is provided with reflection power measuringmeans for measuring power of reflection waves from the process chamber,said matching means is controlled on the basis of measurement valuesobtained by the reflection power measuring means, and the impedance ofthe microwave transmitting means is varied so as to substantiallyeliminate power of the reflection waves.
 4. The apparatus according toclaim 1, wherein the matching means is provided with an electric fieldplane adjusting unit and a magnetic field plane adjusting unit whichhave, respectively, adjustment pipes comprising said first and secondadjustment pipes connected to a rectangular waveguide to correspond toelectric and magnetic field planes perpendicular to each other, andshort plates comprising said first and second plates provided movablyalong the insides of the adjustment pipes.
 5. The apparatus according toclaim 4, wherein the matching means is provided with moving mechanismsfor moving the short plates along the insides of the adjustment pipes.6. The apparatus according to claim 5, wherein the microwavetransmitting means is provided with reflection power measuring means formeasuring power of reflection waves from the process chamber, saidmoving mechanisms are controlled on the basis of measurement valuesobtained by the reflection power measuring means, and the impedance ofthe microwave transmitting means is varied so as to substantiallyeliminate power of the reflection waves.
 7. A plasma process apparatuswhich permits generating microwaves and a magnetic field so as to bringabout electron cyclotron resonance and, thus, to generate a plasma whichis applied to an object to be processed, comprising:microwave generatingmeans for generating said microwaves; microwave transmitting means fortransmitting the microwaves; a process chamber having said objectarranged therein, the microwaves being introduced into said processchamber through said microwave transmitting means; process gas supplymeans for supplying a process gas into said process chamber; andmagnetic field generating means for generating a magnetic field withinthe process chamber, wherein the frequency of the microwave falls withina range between a lower limit of a cutoff frequency determined by theinner diameter of the process chamber and an upper limit of a maximumfrequency at which a standing wave of the microwave does not occur onthe surface of the object, wherein the upper limit and the lower limitin the frequency range of the microwave are 1.5 GHz and 770 MHz,respectively, where the diameter of the object to be processed is 8inches and the microwave oscillates in TM 01 mode within the processchamber, wherein the microwave transmitting means is provided withmatching means for freely varying the impedance of the microwaves in themicrowave transmitting means, and wherein said matching means comprisesfirst and second adjustment pipes connected to said transmitting meanshaving adjustable respective first and second plates disposedperpendicular to each other.
 8. The apparatus according to claim 7,wherein the microwave has a frequency of 915 MHz.
 9. The apparatusaccording to claim 7, wherein the microwave transmitting means isprovided with reflection power measuring means for measuring power ofreflection waves from the process chamber, said matching means iscontrolled on the basis of measurement values obtained by the reflectionpower measuring means, and the impedance of the microwave transmittingmeans is varied so as to substantially eliminate power of the reflectionwaves.
 10. The apparatus according to claim 7, wherein the matchingmeans is provided with an electric field plane adjusting unit and amagnetic field plane adjusting unit which have, respectively, adjustmentpipes comprising said first and second adjustment pipes connected to arectangular waveguide to corresponds to electric and magnetic fieldplanes perpendicular to each other, and short plates comprising saidfirst and second plates provided movably along the insides of theadjustment pipes.
 11. The apparatus according to claim 10, wherein thematching means is provided with moving mechanisms for moving the shortplates along the insides of the adjustment pipes.
 12. The apparatusaccording to claim 11, wherein the microwave transmitting means isprovided with reflection power measuring means for measuring power ofreflection waves from the process chamber, said moving mechanisms arecontrolled on the basis of measurement values obtained by the reflectionpower measuring means, and the impedance of the microwave transmittingmeans is varied so as to substantially eliminate power of the reflectionwaves.
 13. A plasma process apparatus which permits generatingmicrowaves and a magnetic field so as to bring about electron cyclotronresonance and, thus, to generate a plasma which is applied to an objectto be processed, comprising:microwave generating means for generatingsaid microwaves; microwave transmitting means for transmitting themicrowaves; a process chamber having said object arranged therein, themicrowaves being introduced into said process chamber through saidmicrowave transmitting means; process gas supply means for supplying aprocess gas into said process chamber; and magnetic field generatingmeans for generating a magnetic field within the process chamber,wherein the frequency of the microwave falls within a range between alower limit of a cutoff frequency determined by the inner diameter ofthe process chamber and an upper limit of a maximum frequency at which astanding wave of the microwave does not occur on the surface of theobject, wherein the upper limit and the lower limit in the frequencyrange of the microwave are 1.0 GHz and 440 MHz, respectively, where thediameter of the object to be processed is 12 inches and the microwaveoscillates in TE 11 mode within the process chamber, wherein themicrowave transmitting means is provided with matching means for freelyvarying the impedance of the microwaves in the microwave transmittingmeans, and wherein said matching means comprises first and secondadjustment pipes connected to said transmitting means having adjustablerespective first and second plates disposed perpendicular to each other.14. The apparatus according to claim 13, wherein the microwave has afrequency of 915 MHz.
 15. The apparatus according to claim 13, whereinthe microwave transmitting means is provided with reflection powermeasuring means for measuring power of reflection waves from the processchamber, said matching means is controlled on the basis of measurementvalues obtained by the reflection power measuring means, and theimpedance of the microwave transmitting means is varied so as tosubstantially eliminate power of the reflection waves.
 16. The apparatusaccording to claim 13, wherein the matching means is provided with anelectric field plane adjusting unit and a magnetic field plane adjustingunit which have, respectively, adjustment pipes comprising said firstand second adjustment pipes connected to a rectangular waveguide tocorresponds to electric and magnetic field planes perpendicular to eachother, and short plates comprising said first and second plates providedmovably along the insides of the adjustment pipes.
 17. The apparatusaccording to claim 16, wherein the matching means is provided withmoving mechanisms for moving the short plates along the insides of theadjustment pipes.
 18. The apparatus according to claim 17, wherein themicrowave transmitting means is provided with reflection power measuringmeans for measuring power of reflection waves from the process chamber,said moving mechanisms are controlled on the basis of measurement valuesobtained by the reflection power measuring means, and the impedance ofthe microwave transmitting means is varied so as to substantiallyeliminate power of the reflection waves.
 19. A plasma process apparatuswhich permits generating microwaves and a magnetic field so as to bringabout electron cyclotron resonance and, thus, to generate a plasma whichis applied to an object to be processed, comprising:microwave generatingmeans for generating said microwaves; microwave transmitting means fortransmitting the microwaves; a process chamber having said objectarranged therein, the microwaves being introduced into said processchamber through said microwave transmitting means; process gas supplymeans for supplying a process gas into said process chamber; andmagnetic field generating means for generating a magnetic field withinthe process chamber, wherein the frequency of the microwave falls withina range between a lower limit of a cutoff frequency determined by theinner diameter of the process chamber and an upper limit of a maximumfrequency at which a standing wave of the microwave does not occur onthe surface of the object, wherein the upper limit and the lower limitin the frequency range of the microwave are 1.0 GHz and 570 MHz,respectively, where the diameter of the object to be processed is 12inches and the microwave oscillates in TM 01 mode within the processchambers wherein the microwave transmitting means is provided withmatching means for freely varying the impedance of the microwaves in themicrowave transmitting means, and wherein said matching means comprisesfirst and second adjustment pipes connected to said transmitting meanshaving a adjustable respective first and second plates disposedperpendicular to each other.
 20. The apparatus according to claim 19,wherein the microwave has a frequency of 915 MHz.
 21. The apparatusaccording to claim 19, wherein the microwave transmitting means isprovided with reflection power measuring means for measuring power ofreflection waves from the process chamber, said matching means iscontrolled on the basis of measurement values obtained by the reflectionpower measuring means, and the impedance of the microwave transmittingmeans is varied so as to substantially eliminate power of the reflectionwaves.
 22. The apparatus according to claim 19, wherein the matchingmeans is provided with an electric field plane adjusting unit and amagnetic field plane adjusting unit which have, respectively, adjustmentpipes comprising said first and second adjustment pipes connected to arectangular waveguide to corresponds to electric and magnetic fieldplanes perpendicular to each other, and short plates comprising saidfirst and second plates provided movably along the insides of theadjustment pipes.
 23. The apparatus according to claim 22, wherein thematching means is provided with moving mechanisms for moving the shortplates along the insides of the adjustment pipes.
 24. The apparatusaccording to claim 23, wherein the microwave transmitting means isprovided with reflection power measuring means for measuring power ofreflection waves from the process chamber, said moving mechanisms arecontrolled on the basis of measurement values obtained by the reflectionpower measuring means, and the impedance of the microwave transmittingmeans is varied so as to substantially eliminate power of the reflectionwaves.